definition 
The theory of gravitation developed by Albert Einstein (1916) that describes the gravitation
as the spacetime curvature caused by the presence of matter or energy. Mass creates
a gravitational field which distorts the space and changes the flow of time. In other
words, mass causes a deviation of the metric of spacetime continuum from that of
the 'flat' spacetime structure described by the Euclidean geometry and treated in
special relativity. General relativity developed from the principle of equivalence
between gravitational and inertial forces. According to general relativity, photons
follow a curved path in a gravitational field. This prediction was confirmed by the
measurements of star positions near the solar limb during the total eclipse of 1919.
The same effect is seen in the delay of radio signals coming from distant space probes
when grazing the Sun's surface. Moreover, the space curvature caused by the Sun makes
the perihelion of Mercury's orbit advance by 43'' per century more than that predicted
by Newton's theory of gravitation. The perihelion advance can reach several degrees
per year for binary pulsar orbits. Another effect predicted by general relativity
is the gravitational reddening. This effect is verified in the redshift of spectral
lines in the solar spectrum and, even more obviously, in white dwarfs. Other predictions
of the theory include gravitational lensing, gravitational waves, and the invariance
of Newton's gravitational constant.

